Hitherto, 5-phenylisophthalic acid is known to be synthesized through a process employing 1-cyclohexyl-3,5-dimethylbenzene as a starting material and a combination of hydrochloric acid and aluminum chloride serving as a catalyst. However, when this process is employed, selectivity and yield are insufficient, and the used catalyst cannot be recovered. Thus, there is demand for a process which is industrially more advantageous (see Non-Patent Document 1).
Several processes for solving the above problem have already been disclosed. In one process, 3,5-dimethylbiphenyl is synthesized through Suzuki's coupling reaction between bromobenzene and 3,5-dimethylphenylboronic acid in the presence of a transition metal compound such as tetrakis(triphenylphosphine)palladium(0), and the thus-produced 3,5-dimethylbiphenyl is oxidized with an oxidizing agent such as potassium permanganate or potassium chromate, to thereby yield 5-phenylisophthalic acid (see Non-Patent Document 2). In another process, a Grignard reagent produced from 5-bromo-m-xylene and magnesium is reacted with bromobenzene in the presence of a triphenylphosphine-nickel chloride catalyst, to thereby produce 3,5-dimethylbiphenyl, and the produced 3,5-dimethylbiphenyl is oxidized with an oxidizing agent, to thereby yield 5-phenylisophthalic acid (see Patent Document 1). Although the disclosed processes are satisfactory to some extent in terms of selectivity and yield, the processes have problems. In the process employing Suzuki's coupling reaction, an expensive transition metal complex must be used, whereas in the process employing a Grignard reagent, a step for producing the Grignard reagent considerably impairs production efficiency. Both cases are problematic. Therefore, these processes are difficult to employ as industrial production processes.
Non-Patent Document 1:
    Bodroux, Coreaf, C. R. Hebd. Seances Acad. Sci., 186, 1928,Non-Patent Document 2:    Akiyama Ryo and Kobayashi Shue, Angew. Chem. Int. Ed., EN, 40, 18, 2001, 3469-3471Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 9-104679